This invention relates generally to devices which delay the opening of motor starter contacts during the acceleration time of a motor or similar electrical device which acceleration causes high currents on the line to be protected but which currents will become normal after the normal velocity of the running motor has been achieved. This invention relates specifically to plug-in Zener diode type devices which perform this function in a circuit breaker control system.
It is known that some circuit breaker systems have inverse time current overload functions. These functions provide a trip time for the circuit breaker or motor starter which is inversely related to the square of the current flowing in the load to be protected. If the current is above rated current but is relatively low or only incrementally above rated current, it may take many minutes or even hours to cause a trip of the motor starter or circuit breaker. On the other hand, if the current is very high relative to rated current the system may trip within a fraction of a second after the current has begun to flow. It is also known that large motors often have a long acceleration time where very high current flows in the line to be protected. It would be advantageous if an override device could be provided along with the inverse time function to override the inverse time function to prevent tripping of the circuit breaker or motor starter sooner than at least a predetermined fixed time after any overload current is sensed in the line to be protected. It is understood that other circuit protective devices will be employed to detect and protect for catastrophic overcurrent problems such as caused by a direct fault or short circuit.